1. Field of the Invention
This invention relates generally to an earth boring apparatus having sealed lubricant reservoirs enclosing the bearing and seal systems thereof and, more particularly, to means for compensating for internal pressure built up within the lubricant reservoir due to thermal expansion of the lubricant.
2. Brief Description of the Prior Art
In rotatable earth boring apparatus it is common to mount an earth contacting member such as a cutter or a stabilizing roller, on an axle member with bearing means, whether friction or anti-friction disposed therebetween. The working conditions of such apparatus generally are very severe in that the external surrounding enviroment is air-laden with abrasive particles of rock and dust. Thus, it is quite common to seal the bearings from the external environment and, within the sealed bearing environment, provide a lubricant. As is typical in many applications, the seals must also be lubricated to maintain an effective sealing relationship for the effective life of the tool. As these seals are normally located at opposed ends of the axis of the rotating members, and as these rotating members are also quite often disposed on an axis which is vertical or at some rather large angle (i.e. 30.degree. or more) with respect to the horizontal, at least one of these seals tends to be at the greatest elevation of the lubricant reservoir. For such uppermost seal to remain lubricated it is essential to maintain the lubricant reservoir substantially full.
During operation of the earth boring apparatus, the lubricant expands due to its increase in temperature under working conditions, at which time, if the reservoir starts at a condition which is substantially full, expands to a volume to completely fill it and any expansion beyond that causes an internal pressure which is sufficient to breach the seal and expel the lubricant. Upon cooling or when subsequently operating in a less heated condition, contraction of the lubricant will then result in the volume being less than initially filled which likely results in such seal not being fully lubricated, thereby causing deleterious wear thereto. Also, if sufficient oil is expelled from this reservoir, it is not uncommon for the uppermost bearing to also be depleted of oil at least during a portion of its rotation, again with deleterious effects on the life of the bearing.
It is apparent that relieving the pressure with a pressure compensating device that bleeds off the portion of the lubricant during high internal pressure would have the same end result of expelling lubricant from the cavity. U.S. Pat. No. 2,187,037 to L. H. Kirkpatrick shows rotating earth contacting apparatus having a lubricant volume sealed on each end by sealing members which themselves are axially moveable so that if the external pressure is greater than the internal lubricant reservoir pressure, the seals are moved axially into a tighter sealing engagement until such pressures are generally equalized or, in the alternative, if the internal pressure is greater, the end seals are moved outwardly to accommodate this pressure and again tend to equalize internal and external pressure across the seals.